javafaq

Question. Why do we use transient variable? 

Ans:Transient member variable does not serialized with the object. When the class gets de-serialized, this variable gets initialized with a default value of its data type (i.e. zero for integers).

Question. How many types of casting are there in java.

Ans: a)Explicit casting(When u assign superclass object to subclass object)

           Object o = "hello";

           String x;

           x= (String) o;       

       b)implicit casting(when u assign subclass object to superclass)

            String x1 = "hi";

           Object o1;

           o1 = x1;

Question: What's the difference between constructors and other methods?

Answer: Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.

Question: What's the difference between constructors and methods?

Answer:

    Constructors:

        must have the same name as the class name.

        does not return any value.

        called once while making instance of the class

    methods:

        method name differ from class name

        it may return/not return any value.

        can be called many times.

Question: Explain the usage of Java packages.

Answer:

way of organizing the files

helps resolving naming conflicts when classes with the same names.

protect data from being used by the non-authorized classes.

Question: what is difference between "==" and the method "equals()"?

Answer:  '==' : Check the actual object instances are same or not.

         .equals:  check if the contents are equal or not.

Question: what is difference between && and &

Answer:

     &&:if first condition fails then 2nd condition is not checked.

     &:if first condition fails even it checks 2nd condition.

Question: How can a subclass call a method or a constructor defined in a superclass?

Answer: for method :super.myMethod();

        for constructor:super();

Question: How is shallow copy implemented in Java?

Answer: Object cloning.

Question: How would you make a copy of an entire Java object with its state?

Answer: Implement Cloneable interface and override clone() method.

Question: If you're overriding the method equals() of an object, which other method you also need to override?

Answer: hashCode()

Question:What are access modifiers in java?

Answer:

        private: accessible only inside the class

        default: accessible in same package only

        protected: accessible in same package and all subclasses of this class

        public: accessible everywhere

Question: What is polymorphism in java?

Answer: Polymorphism means one name many form. Java implement it in 2 ways.

1) Method overloading(Compile time polymorphism):

   In same class, if name of the method remains common but the number and type of parameters are different, then it is called method overloading in Java.

   public class alpha{

     public int add(int a,int b) {

       return a+b;

     }

     public int add(int a,int b,int c){

       return a+b+c;

     }

     public static void main(String[] args){

       alpha a = new alpha();

       System.out.println(a.add(1,2));

       System.out.println(a.add(1,2,3));

     }

   }

2) Method overriding:

  A method is said to be overridden when one is in parent class and

  another is in child class with the same name, same return type, same

  parameter. It is implemented in two ways:  

    a)through inheritence:

      public class Base{

        public void show(){

          System.out.println("base")

        }

      }

      public class Derive extends Base{

        public void show{

          System.out.println("derive");

        }

        public static void main(String[] args){

          Base b = new Derive();

          b.show();

        }

      }

      b)through java interface :

what is enhanced for loop in java?

for loop before java5(old form):

for (int i=0; i < array.length; i++) {

    System.out.println("Element: " + array[i]);

}

for loop in newer form

for (String element : array) {

    System.out.println("Element: " + element);

}

 1)with old form there is initialization,termination and increment.

 with enhanced for loop we dont require to go for these steps

2) new form increases the clarity and readability.

Why do we go for enhanced for loop even we have iterator?

 when we use iterator there is chances of getting it wrong.

 during nested for loop, with iterator we have to first take the outer variable and

 then we can use it in inner variable.

 for (Iterator i = empList.iterator(); i.hasNext(); ) {

     Emp emp = (Emp) i.next();

     for (Iterator j = deptList.iterator(); j.hasNext(); )

         db_opr.add(emp,j.next);

 }

 but with enhanced for loop we can go straight forward as:

 for (Emp emp : empList)

     for (Dept dept : deptList)

        db_opr.add(emp,dept);

 enhanced for loop has limitations:  

1)The program needs access to the iterator in order to remove the current element.

  2)it is not usable for loops where you need to replace elements in a list or array

  as you traverse it.

  3)it is not usable for loops that must iterate over multiple collections in parallel.